Isolation and characterization of defined populations of immunologically competent cells and their products has facilitated investigations designed to elucidate the mechanism of antigen recognition at the cell surface, the nature of the receptors and the genetic control of such recognition. We have prepared cellular immunoabsorbents using dextranase digestible Sephadex which has allowed the isolation of defined populations of functionally viable lymphocytes according to their surface properties. Specific T cells, B cells and null cells were studied, either separately or in recombined pools both in vivo and in vitro to elucidate their role in the induction of the immune response. These studies have allowed the characterization of the cells involved in mitogen recognition, tumor cell antigen recognition, mediator production and antibody production. Additional studies are underway of the genetic control of the immune response utilizing antibodies of limited heterogeneity and sensitive measures of T and B responses to defined antigens both in man and in the experimental animal.